This invention relates to electrotherapy circuits and more particularly relates to external defibrillators that apply defibrillation shocks to a patient's heart through electrodes placed externally on the patient's body or externally on the patient's heart during surgery.
Normally, electrochemical activity within a human heart causes the organ's muscle fibers to contract and relax in a synchronized manner. This synchronized action of the heart's musculature results in the effective pumping of blood from the ventricles to the body's vital organs. In the case of ventricular fibrillation (VF), however, abnormal electrical activity within the heart causes the individual muscle fibers to contract in an unsynchronized and chaotic way. As a result of this loss of synchronization, the heart loses its ability to effectively pump blood.
Defibrillators produce a large current pulse that disrupts the chaotic electrical activity of the heart associated with ventricular fibrillation and provide the heart's electrochemical system with the opportunity to re-synchronize itself. Once organized electrical activity is restored, synchronized muscle contractions usually follow, leading to the restoration of effective cardiac pumping.
The current required for effective defibrillation is dependent upon the particular shape of the current waveform, including its amplitude, duration, shape (i.e., sine, damped sine, square, exponential decay), and whether the current waveform has a single polarity (monophasic) or has both positive and negative polarity (biphasic). It has been suggested that large defibrillation currents may cause damage to cardiac tissue, however.
It is known to construct an external defibrillator that can sense patient impedance and can set the durations of the first and second phases of a biphasic waveform as a function of the patient impedance. An example of such a defibrillator is described in PCT Patent Publication No. WO 95/05215. Fain et al., U.S. Pat. No. 5,230,336 discloses a method of setting pulse widths of monophasic and biphasic defibrillation waveforms based on measured patient impedance. Kerber et al., "Advance Prediction of Transthoracic Impedance in Human Defibrillation and Cardioversion: Importance of Impedance in Determining the Success of Low-Energy Shocks," 1984, discloses a method of selecting the energy of defibrillation shocks based on patient impedance measured using a high-frequency signal.
It is also known to construct a defibrillator with a safety resistor in the defibrillation path (PCT Patent Publication No. WO 95/05215). Before application of a defibrillation waveform to a patient, a test pulse is passed through the safety resistor while a current sensor monitors the current. If the sensed current is less than a safety threshold representative of a short circuit, the safety resistor is removed and the defibrillation waveform is applied to the patient.
It is known, in an implantable defibrillator, to use a biphasic waveform having a first phase consisting of multiple truncated decaying exponentials that form a sawtooth approximation of a rectilinear shape (Kroll, U.S. Pat. No. 5,199,429). This is accomplished by charging a set of energy storage capacitors and then successively allowing individual capacitors to discharge during the first phase, thereby creating the sawtooth pattern in the output current of the circuit. A more recent patent, Kroll, U.S. Pat. No. 5,514,160, describes a biphasic waveform, in an implantable defibrillator, having a rectilinear-shaped first phase created by placing a MOSFET current limiter in the defibrillation path. This patent states that the grossly non-linear current limiter looks like a small and declining resistance to the capacitor. Also, Schuder et al., "Transthoracic Ventricular Defibrillation in the 100 kg Calf with Symmetrical One-Cycle Bidirectional Rectangular Wave Stimuli" describes the use of biphasic waveforms having rectilinear first and second phases to reverse ventricular fibrillation in calves. Stroetmann et al., U.S. Pat. No. 5,350,403, discloses a waveform having a sawtooth ripple that is formed by periodically interrupting a non-continuous discharge of a charging circuit.